New research raises doubts about whether modern humans and Neanderthals interbred

August 13, 2012

New research raises questions about the theory that modern humans and Neanderthals at some point interbred, known as hybridisation. The findings of a study by researchers at the University of Cambridge suggests that common ancestry, not hybridisation, better explains the average 1-4 per cent DNA that those of European and Asian descent (Eurasians) share with Neanderthals. It was published today, 13 August, in the journal PNAS.

In the last two years, a number of studies have suggested that modern humans and Neanderthals had at some point interbred. Genetic evidence shows that on average Eurasians and Neanderthals share between 1-4 per cent of their DNA. In contrast, Africans have almost none of the Neanderthal genome. The previous studies concluded that these differences could be explained by hybridisation which occurred as modern humans exited Africa and bred with the Neanderthals who already inhabited Europe.

However, a new study funded by the BBSRC and the Leverhulme Trust has provided an alternative explanation for the genetic similarities. The scientists found that common ancestry, without any hybridisation, explains the genetic similarities between Neanderthals and modern humans. In other words, the DNA that Neanderthal and modern humans share can all be attributed to their common origin, without any recent influx of Neanderthal DNA into modern humans.

Dr Andrea Manica, from the University of Cambridge, who led the study said: "Our work shows clearly that the patterns currently seen in the Neanderthal genome are not exceptional, and are in line with our expectations of what we would see without hybridisation. So, if any hybridisation happened - it's difficult to conclusively prove it never happened - then it would have been minimal and much less than what people are claiming now."

Neanderthals and modern humans once shared a common ancestor who is thought to have spanned Africa and Europe about half a million years ago. Just as there are very different populations across Europe today, populations of that common ancestor would not have been completely mixed across continents, but rather closer populations would have been more genetically similar to each other than populations further apart. (There is extensive genetic and archaeological evidence that population in Africa were 'structured'; in other words, different populations in Africa only had limited exchange through migration, allowing them to remain distinct from each other both in terms of genetics and morphology.)

Then, about 350-300 thousand years ago, the European range and the African range became separated. The European range evolved into Neanderthal, the African range eventually turned into modern humans. However, because the populations within each continent were not freely mixing, the DNA of the modern human population in Africa that were ancestrally closer to Europe would have retained more of the ancestral DNA (specifically, genetic variants) that is also shared with Neanderthals.

On this basis, the scientists created a model to determine whether the differences in genetic similarities with Neanderthal among modern human populations, which had been attributed to hybridisation, could be down to the proximity of modern humans in northern Africa (who would have later gone on to populate Europe) to Neanderthals.

By examining the different genetic makeup among modern human populations, the scientists' model was able to infer how much genetic similarity there would have been between distinct populations within a continent. The researchers then simulated a large number of populations representing Africa and Eurasia over the last half a million years, and estimated how much similarity would be expected between a random Neanderthal individual and modern humans in Africa and Eurasia.

The scientists concluded that when modern humans expanded out of Africa 60-70K years ago, they would have brought out that additional genetic similarity with them, making Europeans and Asians more similar to Neanderthals than Africans are on average  undermining the theory that hybridization, and not common ancestry, explained these differences.

Dr Manica added: "Thus, based on common ancestry and geographic differences among populations within each continent, we would predict out of Africa populations to be more similar to Neanderthals than their African counterparts - exactly the patterns that were observed when the Neanderthal genome was sequenced; but this pattern was attributed to hybridisation. Hopefully, everyone will become more cautious before invoking hybridisation, and start taking into account that ancient populations differed from each other probably as much as modern populations do."

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20 comments

This seems to suggest a one-way mixing between the two groups in Africa for 250,000 years, and a one-way migration of almost all of the proto-Eurasians out of Africa instead of some mixing to the south.

Does anyone take away from this that hybridisation did occur. The idea that there was a mass segregation with no interbreeding between the isolated populations existing within the African continent is a more unlikely than interbreeding with another species at a later time. As a matter of fact. Humans have a propensity to attempt mating with many animal species and have depictions of hybrid humans in almost all faiths and traditions. A Neanderthal woman would be exotic and most likely taken as prize by early man.

Hah. We will never prove either position since the underlying question is whether the two subspecies were compatible enough to cross breed successfully. If they were, the odds are long that cross breeding occurred.

Hah. We will never prove either position since the underlying question is whether the two subspecies were compatible enough to cross breed successfully. If they were, the odds are long that cross breeding occurred.

What a ridiculous assertion that has the faint odor of eugenics- Neanderthals and modern humans were a lot closer genetically than say a horse and a donkey which begets a mule, otherwise known as an ASS.

"This is, of course, what was done in the case of the Neandertal [sic - Hawks' preferred spelling] genome -- a specific population model was significantly favored by the data, and alternatives that did not include population mixture were demonstrated to be so unlikely as to be essentially impossible."

More interesting in that PNAS is the new estimate of extant homininae mutation rates, which surprisingly by fecal et cetera DNA derives chimp generation ages on pair with humans @ 25 years and push back the chimp/human and gorilla/human splits a few million years. (Under the assumption of constant rates.)

Neanderthals and modern humans were a lot closer genetically than say a horse and a donkey which begets a mule, otherwise known as an ASS.

Don't make an ass of yourself, now--'ass' is synonymous with 'donkey', not with 'mule'.

@Torbjorn_Larsson_OM

"This is, of course, what was done in the case of the Neandertal [sic - Hawks' preferred spelling]

Not just Hawks'--it reflects the modernization of German spelling. The 'h' in 'thal' (and various other words) has not been pronounced in a long time, and it has been dropped from such terms as needless.

@ Telekinetic:"This is worth reading, and far more credible than John Hawks".

It is worth reading, but you must have skimped on Hawks. He is a paleontologist who is _a Neanderthal specialist_. As he notes, he started out with fossils and is now, expertly because he has published a lot, looking at genetics of humans and neanderthals.

He can be very cautious in places, and I find myself referencing him a lot.

@ Shakescene21:

It is basically population genetics, which is the science that joined evolution with genetics in the 30's (IIRC). Nothing new or mysterious.

YMMV of course, these methods and results needs to be checked over and over as always.

@ Silverhill:

I know, and I was considering using it myself a few years back. But a) it was the historical spelling at the time, b) " the laws of taxonomy retain the original spelling at the time of naming." [Wp]

Actually, I now remember I _was_ using it a while. But as I got wiser on taxonomy and it having such rigid rules, I changed back.

Now one may have discussion about informal use, change from without vs within et cetera. But in this case I want to be easily understood. It's hard enough when I start to make the distinction between fossil species, biological species and "genome species" which are all involved in these phylogenies.

Oh well. Back to hominins. Wonder if the genes introgressed in the human population as opposed to the Neanderthal not because it was larger (early meeting in Asia right after out-of-Africa), but because the bottleneck of humans happened about then and the survivors were the ones procreating like rabbits?

Norwegians are proud to be descended from Eric the Red, so there's no shame in having a little Neanderthal in you. One should be wary of red-headed females, though, because of their hair-trigger temper, a remnant of a Neanderthal past.

A suspicious aspect of this study is that it assumes that Neanderthals and Moderns diverged 300,000 to 350,000 years ago. More knowledgeable researchers date the divergence at 400,000 to 800,000 years ago. http://phys.org/n...ged.htmlI wonder if a longer divergence time would change the conclusions of the model. More importantly, a divergence time of 600,000 years ago instead of 325,000 would seriously undermine the shakiest assumption of the analysis -- namely that the gene flow of the African population was exclusively from Central Africa to North Africa for hundreds of thousands of years.